1. Field of the Invention
The present invention relates generally to a manufacturing method of semiconductor devices, and more particularly to a method for manufacturing or fabricating semiconductor devices by use of dry etching techniques.
2. Related Art
One prior known approach to formation of angular rate sensors with piezoelectrically vibratory gyros by use of silicon micro-machine technology has been disclosed in, for example, Published Unexamined Japanese Patent Application (PUJPA) No. 6-147903. More practically, through-going holes are formed by etching in a thin-walled or "reduced-thickness" portion of a silicon substrate, providing a vibrator with a cantilever structure. In this case, the vibrator is formed by performing etching treatment from the opposite surfaces of the silicon substrate. Note that the etching treatment employed here may be generally classified into two categories: (1) fabrication within the surface of silicon substrates (areal etching), and (2) fabrication along the thickness (thickness etching). The silicon-substrate areal fabrication (areal etching) is to directly etch the substrate by photolithography technology with a resist pattern being used as a mask in a manner similar to those of ordinary semiconductor devices; this type of etching is high in degree of freedom of shape while allowing its etching conditions to preferably accord with those of the methods for fabrication of standard semiconductor devices. On the other hand, in the along-the-thickness fabrication (thickness etching), it remains difficult to attain direct etching with a resist pattern being as a mask therefor; alternatively, the shape is to be determined either by utilizing one of dummy or "sacrifice" layer preformed along the thickness of a wafer, etching stopper layer and PN junction or by controlling the etching amount in time management schemes. In this case, the etching amount (area and depth) is relatively greater as compared with ordinary fabrication of semiconductor devices while the etching conditions tends to be severe if priority is given to throughput.
Due to the above-mentioned characteristics, the areal etching is typically designed so that the top surface is first subject to dry etching, whereas the thickness etching causes the bottom surface to be first processed by wet etching techniques. Accordingly, when a silicon substrate is etched from its both surfaces in order to form an intended vibrator, areal etching is effected with respect to a silicon substrate 60 thereby defining recess portions 61 as shown in FIG. 18. Thereafter, as shown in FIG. 19, thickness-etching is then effected with respect to the silicon substrate 60 forming a recess portion 62 to thereby let the recesses 61 be through-going holes. Alternatively, as shown in FIG. 20, a silicon substrate 63 is subject to the thickness-etching forming a recess portion 64; then, as shown in FIG. 21, the silicon substrate 63 is subject to areal etching treatment defining several through-holes 65.
A comparison of these two etching schemes is as follows. With the etching method shown in FIGS. 18 and 19, during the thickness etching, etchant liquid or gas tends to locally enter and leak through the recesses 61 as completed at termination of the areal etching process--namely, the resulting through-holes defined--resulting in unexpected etching and/or undesired surface configuration being easily occurred on the substrate surface and on the side surfaces of each recess 61. On the other hand, with the method of FIGS. 20 and 21, this offers an advantage that elements to be formed on the top surface and their associated lead wires can be free from the thickness-etching of strict conditions because this method is arranged so that the thickness-etching is effected from the bottom surface to form the reduced-thickness portion 66 and thereafter the areal etching is performed from the top surface thereof. For this reason, the method shown in FIGS. 20 and 21 may be employed for the manufacture of sensors.
However, where dry etching is effected with respect to the reduced-thickness portion 66 by use of the method of FIGS. 20-21, a local temperature increase will arise for the following reasons. As shown in FIG. 22, the reduced-thickness portion 66 formed by thickness-etching is less in heat transmission area than non-etched portions; therefore, heat hardly flows toward the side of a base plate 67. Also, as shown in FIG. 23, when the areal etching is carried out, the heat transmission area decreases with progress of etching thereby rendering more difficult the flow of heat toward base plate 67.
The local temperature increase badly behaves to increase the rate of reaction of the silicon substrate (wafer) with etching gas and radicals. As a result, the side-etching rate increases while the silicon substrate 63 and reduced-thickness portion 66 can deform due to application of thermal stress causing an etching mask to be rough and overetched at edges thereof, which in turn leads to a decrease in accuracy of areal etching treatment.
A technique of cooling the substrate by supplying thereto chosen coolant gas to the substrate being placed on the supporting table in a chamber during etching has been disclosed in PUJPA Nos. 1-251735, 6-112302, 7-249586 and others. It might be considered that the technique disclosed therein is employed for supplying the substrate with coolant gas during etching for formation of through-holes. However, with such an arrangement, when through-holes are completed penetrating substrate, such coolant gas attempts to enter or "invade" the inside of the etching chamber making it impossible to attain regular etching treatment, which in turn results in a decrease in fabrication accuracy.
As the cooling method of the substrate (wafer) for use in standard dry etching apparatus, another method is known which forces the substrate to be in close contact at its bottom surface with the stage which has a cooling function. This method is disclosed, for example, in PUJPA No. 8-165571, wherein chosen refrigeration medium or coolant is sealed on the upper surface of the support table by using a flexible sheet while placing the substrate on this sheet with an elastic rubber member being laid therebetween, thereby cooling the substrate by the coolant during etching. Presumably, this technique is used to attain the etching for formation of through-holes. However, with this approach, while it is required that the substrate be disposed above the coolant-sealing flexible sheet with the rubber member being interposed therebetween after formation of the recess portion 64 in the silicon substrate 63 of FIG. 20, it remains unable to dispose the coolant-sealing flexible sheet and rubber member in such a way as to almost completely fill the inside of recess 64 of silicon substrate 63 shown in FIG. 20, which results in a decrease in cooling efficiency with the fabrication accuracy degraded.